This invention relates to an amplifying solid-state image sensor for amplifying an electric signal charge obtained by a photodiode in units of a pixel to obtain an electric signal, in particular, an amplifying solid-state image sensor intended to improve the structure of pixels, and a manufacturing method thereof.
In recent years, an amplifying solid-state image sensor having high sensitivity derived from an amplifying function provided to each pixel has attracted attention. Such an amplifying solid-state image sensor changes a potential at an electric signal charge accumulation section by using electric signal charges obtained by photoelectric conversion. The change of the potential at the electric signal charge accumulation section modulates an amplifying transistor in each pixel, which amplifies the electric signal charge. This amplifying function allows the amplifying solid-state image sensor to have high sensitivity, and an operation such as increasing the number of the pixels or reducing the pixels in size due to the reduction of the image size can be suitably performed in such a device.
Each pixel in the amplifying solid-state image sensor as described above generally comprises such components as a photodiode for performing the photoelectric conversion; a reset transistor for resetting the potential of the photodiode; an amplifying transistor for amplifying an electric signal charge; and a wiring connecting a selection transistor for selecting a desired line or a charge coupled photodiode, with a gate of the amplifying transistor. When the photoelectrically converted electric signal charge needs to be temporarily stored, the image sensor is provided with a storage diode in a region different from that of the photodiode, and with a transfer transistor arranged between the photodiode and the storage diode.
The above-mentioned pixel structure of the amplifying solid-state image sensor has two active regions on a semiconductor substrate which have the same impurity concentration as that of the semiconductor substrate: an active region relating to the photoelectric conversion, i.e., an active region in which the photodiode, the transfer transistor, the storage diode, and the reset transistor are formed; and an active region in which an amplifying transistor is formed. Due to this structure, however, the following problems occur:
In order to decrease noise (a dark current) generated in the photodiode during the photoelectric conversion, it is remarkably effective to suppress the influence of an electric current generated in the Si--SiO.sub.2 interface on the surface of the semiconductor substrate. To suppress the influence of the electric current, a so-called pinned photodiode structure is generally used: the pinned photodiode structure is obtained by doping a p-type impurity having a high concentration into the region near the Si--SiO.sub.2 interface in the photodiode section.
In the meantime, in order to decrease the amplifying transistor in size to satisfy the request for reduction of a pixel size, the impurity concentration of the semiconductor substrate at a region where the amplifying transistor is formed needs to be increased in accordance with the general scaling rule. The amplifying transistor and the photodiode are, however, both formed on the semiconductor substrate so as to have the same impurity concentration as that of the semiconductor substrate, as described before. If the impurity concentration of the substrate is increased to decrease the amplifying transistor in size, the impurity concentration of a region of the substrate, on which the photodiode is formed, is also naturally increased. On a substrate with such a high impurity concentration, the pinned photodiode cannot be easily formed for the reason suggested below.
The semiconductor substrate and the photodiode have conductivity types opposite to each other, and thus the impurity concentrations of the substrate and the photodiode region offset each other, with the result that the photodiode impurity region will be reduced in impurity concentration and area. With such a structure, a sufficient capacity for storing the electric charge cannot be attained securely. When the photodiode impurity region is formed in the semiconductor substrate having a low impurity concentration, no problems will occur. In a device having a substrate with a high impurity concentration, however, a considerably big problem may occur.
As described above, it is necessary to increase the impurity concentration in the substrate in order to reduce the size of the amplifying transistor in the conventional amplifying solid-state image sensor in accordance with the general scaling rule. If the impurity concentration in the substrate is increased, however, the low dark-current characteristics of the photodiode may be deteriorated.